14.2 Sense organs

2026 Syllabus Objectives

  1. Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals
  2. Identify in diagrams and images the structures of the eye, limited to: cornea, iris, pupil, lens, retina, optic nerve and blind spot
  3. Describe the function of each part of the eye, limited to:
    • (a) cornea – refracts light
    • (b) iris – controls how much light enters the pupil
    • (c) lens – focuses light on to the retina
    • (d) retina – contains light receptors, some sensitive to light of different colours
    • (e) optic nerve – carries impulses to the brain
  4. Explain the pupil reflex, limited to changes in light intensity and pupil diameter
  5. Explain the pupil reflex in terms of the antagonistic action of circular and radial muscles in the iris
  6. Explain accommodation to view near and distant objects in terms of the contraction and relaxation of the ciliary muscles, tension in the suspensory ligaments, shape of the lens and refraction of light
  7. Describe the distribution of rods and cones in the retina of a human
  8. Outline the function of rods and cones, limited to:
    • (a) greater sensitivity of rods for night vision
    • (b) three different kinds of cones, absorbing light of different colours, for colour vision
  9. Identify in diagrams and images the position of the fovea and state its function

What are Sense Organs? 🔍

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Sense organs are groups of receptor cells that respond to specific stimuli in the environment. These specialized structures allow organisms to detect changes in their surroundings and respond appropriately.

Key Characteristics of Sense Organs

  • Contain receptor cells or specialized sensory neurone endings
  • Respond to specific stimuli such as:
    • Light (detected by eyes)
    • Sound (detected by ears)
    • Touch (detected by skin)
    • Temperature (detected by skin)
    • Chemicals (detected by nose and tongue)

Important: Each sense organ is specialized to detect a particular type of stimulus. For example, the eye contains receptor cells in the retina that are sensitive to light.

Structure of the Eye 👁️

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The eye is a complex sense organ designed to detect light and create visual images. Understanding its structure is essential for understanding how it functions.

Protection and Maintenance

The eye is well-protected and maintained by several features:

  • Set in a bony socket in the skull for protection
  • Only the front of the eye is not surrounded by bone
  • Filled with fluid to maintain its shape
  • Covered by a thin, transparent membrane called the conjunctiva
  • Surface kept moist by fluid from tear glands
  • Tear fluid contains lysozyme, an enzyme that kills bacteria
  • Eyelids blink to wash fluid across the eye

Key Structures of the Eye

The eye contains several important structures, each with specific functions:

StructureDescription
ConjunctivaThin, transparent membrane covering the front of the eye for protection
CorneaTransparent layer that refracts (bends) light rays entering the eye
PupilCircular gap in the middle of the iris through which light passes
IrisColoured part of the eye; contains muscles that control pupil size
LensTransparent structure that changes shape to focus light onto the retina
Ciliary muscleCircle of muscle surrounding the lens; controls lens shape
Suspensory ligamentsStrong, inelastic fibres that hold the lens in position
RetinaTissue at the back of the eye containing light receptor cells
Optic nerveNerve that carries electrical impulses from the retina to the brain
Blind spotPart of the retina where the optic nerve leaves; contains no receptor cells

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